Land vibrators are known which include a base plate in contact with the earth, a reaction mass, and a linear actuator for reciprocating the reaction mass relative to the base plate.
U.S. Pat. No. 3,745,885 discloses a conventional hydraulic vibrator with a cylindrical piston and cylinder. More particularly, the vibrator is comprised of a double acting drive piston slidably received within a cylinder formed in a reaction mass. When hydraulic fluid is introduced into the cylinder alternately on opposite faces of the piston, the reaction mass is forced into reciprocal motion.
Marine vibrators also are known which employ a reaction mass in the same manner as land vibrators, where a reciprocating force is imparted to a single baseplate in contact with the water. Other marine vibrators, such as that disclosed in U.S. Pat. No. 3,482,646, substitute the reaction mass of a land vibrator with a second baseplate substantially identical to the first. A flexible seal between the baseplates allows them to move with respect to each other without compromising the watertight integrity of the assembly. Each baseplate works against the mechanical and reactive impedance of the other one to form a pair of acoustic projector surfaces experiencing equal and opposite forces, and therefore moving in opposite directions.
Other known actuators, some of which are not applicable as seismic vibrators, are described in the following patent summaries:
U.S. Pat. No. 3,172,338 discloses a hydropneumatic actuator which acts as a tool and die manipulator, rather than a vibrator. The actuator includes three coaxial, concentric, and coextensive cylinders, with each cylinder having in slidable relation therewith a piston. Not all pistons have double ended rods. If equal pressure is applied to each side of the piston, therefore, an unbalanced force is generated, and hence an asymmetrical displacement of stroke occurs.
U.S. Pat. No. 4,143,736 discloses a seismic transducer for generating waves in an elastic medium. The transducer includes a reaction mass with parallel cylinders formed therein, and a piston member slidably received in each cylinder. Each piston includes a double ended rod, with one rod end connected to an energy coupling plate in contact with the elastic medium to be vibrated. A pressurized fluid supply, pressurized fluid storage, manifold and servo valve are used to introduce hydraulic fluid alternately to opposite sides of the pistons to induce a reciprocal motion into the reaction mass.
U.S. Pat. No. 4,178,838 discloses a seismic energy vibrator which includes a reaction mass with parallel cylinders into which pistons with a double ended rod are reciprocally received. One rod end of each piston is attached to an energy coupling plate in contact with the earth. When hydraulic fluid is alternately introduced to opposite sides of the pistons, the reaction mass is forced into reciprocal motion. One aspect of the disclosed invention is that hydraulic flow porting is simplified to provide only a single hydraulic flow passage for each piston rod, thereby improving structural integrity.
U.S. Pat. No. 4,386,889 discloses a positive displacement pump having a plurality of slidably reciprocating, concentric, annular pistons mounted between parallel walls of the pump. When the pistons are reciprocated in a predetermined and controlled sequence, the fluid is caused to flow through the pump. The purpose of the apparatus is to pump liquid, not to produce vibrations.
U.S. Pat. No. 4,424,012 discloses an in-line pump having a cylinder in which an annular piston is reciprocally driven to cause fluid to flow along a fluid carrying line. The piston rods are single ended, and thus produce asymmetrical forces on the up and down strokes. The pump is valved in such a way that an applied reciprocating force on the single-ended piston rods produces a unidirectional flow of oil in the inlet and outlet ports. In the present invention, a reciprocating flow of pressurized fluid produces a symmetrical force on the piston which is transmitted through double ended piston rods to vibrate external masses.
U.S. Pat. No. 4,608,675 discloses a toroidal piston in a toroidal enclosure with only one chamber formed between the base of the piston and the base of the enclosure. The piston is of sufficient size to act as the reaction mass. The rapid release of air into the chamber transmits an impulse into the ground as the baseplate of the enclosure is pushed against the inertia of the piston to produce seismic energy. No coherent vibrational energy, however, is produced. That is, no reciprocating vibrational force is produced.
U.S. Pat. No. 4,691,803 discloses a seismic energy generating system including a base plate in contact with the earth, a master cylinder filled with water that is mounted on the base plate, and a receiving cylinder within the master cylinder into which a projectile is fired to generate a hydraulic force that is coupled through the base plate to the earth. This device is another impulse generator, and does not produce a reciprocating vibrational force.
U.S. Pat. No. 4,939,983 discloses a fluid pressure operated positioning apparatus which includes a ring enclosure having coaxial inner and outer walls, and an annular piston surrounding the inner enclosure wall. Axially parallel piston rods are connected to the annular piston, and extend in a sealed manner through one or both enclosure end caps. The positioning apparatus is used to apply a continuous force between two objects, generate rotational motion, control a robotic manipulator arm, or position a workpiece or table for machining or grinding. No reciprocating vibrational force is produced.
U.S. Pat. No. 5,189,263 discloses a portable geophysical energy source including an earth-coupling element, a seismic energy source connected to the earth-coupling element, a lower water container placed between the seismic energy source and the earth coupling element to act as a hold-down mass, and an upper water container placed above the seismic energy source to act as a reactance mass. The hydraulic actuator used in the system is of a conventional circular piston in a cylinder design.
U.S. Pat. No. 5,360,951 discloses a seismic energy source which includes a first plate resting on the earth's surface, one or more piezoelectric transducers mounted on the first plate to convert electrical energy into mechanical vibrating energy, and a second plate resting on top of the transducers and anchored to the earth by an earth clamping mechanism. When electrical energy is imparted to the transducers, a mechanical vibratory motion is imparted into the earth through the first plate.
U.S. Pat. No. 5,410,946 discloses a die press with a dual stage hydraulic actuator having a single cylinder body with a pressure chamber in which a first piston and a second piston are independently slidably inserted. Each piston includes plural single ended piston rods which protrude through only one end of the cylinder, and which are equally spaced circumferentially. The piston rods of the first piston are surrounded by the piston rods of the second piston. The cylinder body has a center column which allows the first piston to separate first and second pressure chambers, and the second piston to separate second and third pressure chambers. The first and second pistons are advanced by introducing pressured oil into the first pressure chamber, and then the second pressure chamber. The pressurized oil in the second pressure chamber acts as a rigid fluid connector to transmit the movement of the first piston to the second piston. Asymmetric rather than symmetric forces are produced on either side of the pistons given the same fluid pressure.
U.S. Pat. No. 5,701,801 discloses a mechanically redundant actuator with structurally redundant members, which attach the actuator body between a stationary anchor point and a movable control surface of an aircraft. The actuator is comprised of a single circular piston within a cylinder. The piston rods are double ended.
None of the above prior art describes an actuator in which a reciprocating flow of fluid produces a symmetrical force on the piston which is transmitted through double ended piston rods in a manner to distribute a reciprocating vibrational force over a large surface area of the mass to be vibrated, thereby reducing likelihood of inducing strain, stress, or harmonics in the mass.